General methods of detecting and adjusting the focus of a camera using a light beam having passed through a photographing lens are a contrast detection method and phase-difference detection method. The contrast detection method is popular in video cameras and digital still cameras, and uses an image sensor as a focus detection sensor. This method pays attention to a signal output from the image sensor, especially information (contrast information) of a high-frequency component. A photographing lens position where the evaluation value of the contrast information maximizes is set as an in-focus position. However, the contrast detection method, also called a hill-climbing detection method, is not suitable for a high-speed focus detection operation. This is because the evaluation value is obtained while slightly moving the focus position of the photographing lens. The focus position needs to be moved until it is found out that the evaluation value was maximum.
Focus detection by the phase-difference detection method is adopted in many single-lens reflex cameras. This technique is most contributed to practical use of AF (Auto Focus) single-lens reflex cameras. The AF in the phase-difference detection method is generally achieved by a focus detection means formed from a secondary imaging optical system. The focus detection means includes a pupil-dividing means for splitting a light beam having passed through the exit pupil of the photographing lens into two regions. The two split light beams are respectively received by paired focus detection sensors via the secondary imaging optical system. The defocus amount of the photographing lens is directly obtained by detecting the shift amount between signals output in accordance with the light receiving amounts, i.e., a relative positional error in the pupil-dividing direction. Once the focus detection sensor executes an accumulation operation, the defocus amount and direction can be attained at once. This enables a high-speed focus adjustment operation.
According to this focus detection method, a photographing light beam guided to the image sensor and a focus detection light beam guided to the focus detection means out of a light beam having passed through the photographing lens differ from each other. Owing to aberrations (e.g., spherical aberration) of the photographing lens, optimum image plane positions do not match each other. It is known to store in advance the difference between optimum image plane positions arising from the difference between light beams, and correct a focus detection result in focus detection. In a camera system including a single-lens reflex camera and a plurality of photographing lenses interchangeably mounted on the camera, each photographing lens generally stores in advance a correction value corresponding to the difference between optimum image plane positions. In focus detection, the photographing lens sends the correction value to the camera, implementing high-precision focus detection. Thus, the difference between optimum image plane positions can be appropriately corrected regardless of a photographing lens mounted on the camera. For example, Japanese Patent Laid-Open No. 63-172110 discloses this technique.
There is also proposed a technique of adding a phase-difference detection AF function to an image sensor. This technique achieves high-speed AF while the user confirms an image in real time on a display means such as a rear liquid crystal display. For example, in Japanese Patent Laid-Open No. 2000-156823, a pupil-dividing function is added to some light receiving elements (pixels) of an image sensor by decentering the sensitive regions of light receiving portions from the optical axis of an on-chip microlens. These pixels are used as focus detection pixels and arranged between image sensing pixels at predetermined intervals to perform phase-difference focus detection. No image sensing pixel exists at a portion where a focus detection pixel is arranged. Image information at this portion is generated by interpolation using information of peripheral image sensing pixels. In this example, phase-difference focus detection can be done on the image sensing plane, achieving high-speed, high-precision focus detection.
Recently, it is examined to add the phase-difference detection AF function of the image sensor to a camera with phase-difference detection AF using the secondary imaging optical system. This camera can execute phase-difference AF by using the secondary imaging optical system in an object observation state via an optical viewfinder and by using the image sensor in an object observation state via a display means such as a rear liquid crystal display. High-speed AF can be done in the two observation states using the optical and electronic viewfinders.
However, this camera suffers the following problem.
In phase-difference AF using the image sensor, similar to phase-difference AF using the secondary imaging optical system, a focus is basically detected using light beams having passed through two different regions out of a light beam having passed through the exit pupil of the photographing lens. Since the photographing light beam and focus detection light beam differ from each other, the camera requires a correction value corresponding to the difference between optimum image plane positions. According to phase-difference AF using the secondary imaging optical system, the photographing lens stores in advance a correction value corresponding to the difference between optimum image plane positions, so the camera can execute high-precision AF. To the contrary, phase-difference AF using the image sensor does not consider a correction value corresponding to the difference between optimum image plane positions, and the camera cannot perform high-precision AF.
As a measure, a correction value based on an optimum image plane position may be prepared in advance in phase-difference AF using the image sensor. However, in a camera system in which the camera allows interchanging a plurality of photographing lenses, especially a photographing lens released in the past does not hold a correction value corresponding to the difference between optimum image plane positions. It is necessary to store a correction value in the camera for each photographing lens and ensure a large storage area at high cost.